There are many medical diagnostic tests that require a fluid, for example without limitation, blood (sometimes referred to as whole blood, in order to differentiate blood from serum and plasma), serum, plasma, cerebrospinal fluid, synovial fluid, lymphatic fluid, calibration fluid, and urine. With respect to blood, a blood sample is typically withdrawn in either an evacuated tube containing a rubber septum (a vacutainer), or a syringe, and sent to a central laboratory for testing. The eventual transfer of blood from the collection site to the testing site results in inevitable delays. Moreover, the red blood cells are alive and continue to consume oxygen during any delay period, which in turn changes chemical composition of the blood sample in between the time the blood sample is obtained and the time the blood sample is finally analyzed.
One example of a blood analysis technique that is affected by the aforementioned sources of error is co-oximetry. Co-oximetry is a spectroscopic technique that can be used to measure the different Hemoglobin (Hb) species present in a blood sample. The results of co-oximetry can be further evaluated to provide Hb Oxygen Saturation (Hb sO2) measurements. If the blood sample is exposed to air, the Hb sO2 measurements are falsely elevated, as oxygen from the air is absorbed into the blood sample. Co-oximetry also typically requires hemolyzing of the red blood cells (hemolysis), using a sound generator, to make the blood sample suitable for spectroscopic measurement. Hemolysis can also be accomplished by chemical means. Parameters that can be measured in blood by spectroscopic techniques (or spectroscopy, sometimes referred to as spectrometry) are limited by the amount of electromagnetic radiation (EMR) absorbed by the analytes measured. For example, without limitation, hydrogen ions (which determine pH) and electrolytes (sodium, potassium, chloride and bicabonate) do not absorb EMR in the approximate wavelength range of about 300 nm to 2500 nm. Therefore, if this wavelength range is used to conduct spectroscopic measurements of Hb species for example, then these important parameters, i.e., hydrogen ions and electrolytes, must be measured by other means.
Another example of a blood analysis technique that is affected by the aforementioned sources of error is blood gases. Traditionally, blood gas measurement includes the partial pressure of oxygen, the partial pressure of carbon dioxide, and pH. From these measurements, other parameters can be calculated, for example, Hb sO2. Blood gas and electrolyte measurements usually employ biosensors. Bench-top analyzers are available, which (1) measure blood gases, (2) perform co-oximetry, or (3) measure blood gases and perform co-oximetry in combination. Some combinations of diagnostic measurement instruments also include electrolytes, making such bench-top analyzers even larger. Because these instruments are large and expensive, they are usually located in central laboratories. Biosensor technology is also limited by the blood parameters it can measure. To the inventor's knowledge, biosensors are not currently available for measuring the Hb species measured by co-oximeters.
Preferably, blood gases and co-oximetry are measured in arterial blood collected in a syringe, since arterial blood provides an indication of how well venous blood is oxygenated in the lungs. There are many benefits to providing these blood tests near or at the point of care of patients, but these are usually limited by the size and cost of the diagnostic measurement instruments. As a non-limiting example, assessment of the acid-base status of a patient requires both the measurement of hemoglobin (Hb) species in the blood and the blood pH. Therefore, there is a need for small portable meters, which combine spectroscopic technology with biosensor technology.